It is known that compositions comprising a mixture of a nitrile-containing highly saturated copolymer rubber obtained by hydrogenating the unsaturated bonds of the butadiene portions of an acrylonitrile-butadiene copolymer rubber, and an ethylene-propylene copolymer rubber can yield vulcanized products having excellent weather resistance and processability, having good heat resistance and oil resistance, and hence suitable for use as seals (for example, Japanese Patent Laid-Open Nos. 283639/'86 and 124951/'90).
Although the vulcanized products of these rubber compositions are rubber materials suitable for applications requiring excellent weather resistance and processability in combination with high strength and oil resistance, especially for automobile parts and the like, much has been unknown as to whether they can be used for applications other than automobile parts and the like. For example, it has been believed that they do not have satisfactory performance when used for applications in which they come into long-term contact with hot water containing bleaching powder capable of generating hypochlorous acid, such as hot-water seals used in hot-water generators for home or business use.
In such hot-water generators, ethylene-propylene copolymer rubber and acrylonitrile-butadiene copolymer rubber (NBR) have conventionally been used as rubber materials for hot-water seals. However, in the case of ethylene-propylene copolymer rubber, its long-term use tends to cause the release of carbon black used as the reinforcing agent. Moreover, NBR has the disadvantage that the rubber progressively hardens to produce a large number of cracks in the rubber surface. Consequently, it has been desired to develop an improved technique.
The present inventors have now discovered that, when a mixture of a nitrile-containing highly saturated copolymer rubber and ethylene-propylene copolymer rubber is vulcanized and then immersed in hot water containing bleaching powder for a long period of time, its change in volume is unexpectedly slight. The present invention has been completed on the basis of this discovery.